Diesel engines are used in passenger cars and, to a greater degree, in trucks and buses; however, because of their wide use, their emissions can have a significant, negative environmental impact. Also, even modest uses in enviornments such as mining, can be a problem. Interest is strong in developing means to operate diesel engines while controlling their most troublesome characteristics.
In a July 1994 paper entitled "Particulate Emission Control of Diesel-Fueled Vehicles", B. I. Bertelsen presented a "Status Report" discussing the coming regulations, accomplishments to date and shortcomings of current technology. He notes that the art has developed catalytic oxidizers for diesel engines in the form of traps and pass-through catalysts. Both types of catalytic structures enable operation of the diesel engines with reduced emission of particulates. However, as currently available, neither type can be effective over long periods of operation. The traps become plugged, and it is difficult to remove the particulates they collect. Additionally, the catalysts tend to become inactive for a variety of reasons and tend to oxidize SO.sub.2 to SO.sub.3. This latter problem also affects catalytic devices of the pass-through type. Increasing precious metal content in an effort to improve durability increases the problem of oxidation of SO.sub.2 to SO.sub.3.
Diesel particulates, their effect and control, are at the center of much concern and controversy. Their chemistry and environmental impact present complex issues. Very generally, the diesel particulate matter is principally solid particles of carbon and metal compounds with adsorbed hydrocarbons, sulfates and aqueous species. Among the adsorbed species is a soluble organic fraction (SOF) known to contain aldehydes and polycyclic aromatic hydrocarbons (also called PAH's). Some of these organics have been reported to be potential carcinogens. Unburned hydrocarbons are related to the characteristic diesel odor and include aldehydes such as formaldehyde and acrolein. The aldehydes, like the carbon monoxide, are the products of incomplete combustion.
It is not just these organics which are of concern. In one study, diesel particulates were tested along side TiO.sub.2 and carbon without any adsorbed hydrocarbons. (U. Heinrich, et al, "Tierexperimentelle Inhalationsstudien Zur Frage der Tumorinduzierenden Wirkung von Dieselmotorabgasen und zwei Teststauben", Oklolgische Forschung BMFT/GSF, Munich, 1992) The reporters determined that all species tested showed carcinogenic tendency. Until further work clarifies this matter, it would be prudent to look for systems which could control particulates of all composition.
Unfortunately, increasing the recovery of particulates by utilizing a trap device, can decrease fuel economy by increasing exhaust back pressure due to particulate buildup within the trap. Moreover, the various pollutants seem to be interrelated, with reduction of one sometimes increasing levels of another. By modifying combustion to achieve more complete oxidation, decreases can be achieved for pollutants resulting from incomplete combustion, but NO.sub.x is typically increased under these conditions.
NO.sub.x, principally NO and NO.sub.2, contributes to smog, ground level ozone formation and acid rain. NO is produced in large quantities at the high combustion temperatures associated with diesel engines. The NO.sub.2 is formed principally by the post oxidation of NO in the diesel exhaust stream. Several attempts have been made to reduce NO.sub.x, such as by retarding engine timing, exhaust gas recirculation, and the like; however, with current technology, there is a tradeoff between NO.sub.x and particulates. When NO.sub.x, is reduced, particulate emissions increase. And, as noted, conditions favoring low emissions of NO.sub.x often favor production of increased levels of CO and HC. It would be desirable to have a catalyst system which control particulate emissions to a level permitting the use of known means to reduce NO.sub.x.
In "A New Generation of Diesel Oxidation Catalysts", Society of Automotive Engineers (SAE Paper No. 922330, 1992), R. Beckman, et al. assert that the technical challenge is to find a catalyst which selectively catalyzes the oxidation of carbonaceous components at low exhaust temperatures typical of diesels operating at partial load, and that does not oxidize sulfur dioxide or nitrogen oxide at high load temperatures. They described tests (without specifically identifying the catalysts) studying the aging of platinum-catalyzed cordierite honeycomb traps, and concluded, inter alia, that the aging was related to adsorption of sulfur and this depended on both the sulfur content of the fuel and the phosphorous content of the lubricating oil. With control of both of these, aging could be slowed. However, sulfur will remain in diesel fuels, even with planned reduction to 0.05%, and there will continue to be a need for a means to maintain the activity of catalysts for reducing emissions of particulates, and preferably also carbon monoxide and unburned hydrocarbons.
In "Control of Diesel Engine Exhaust Emissions in Underground Mining", 2nd U.S. Mine Ventilation Symposium, Reno, Nevada, Sep. 23-25, 1985, at page 637, S. Snider and J. J. Stekar report that precious metal catalysts in a catalytic trap oxidizer and a "catalyzed Corning trap" were effective in the capture of particulate matter, but both systems increased the conversion of SO.sub.2 to SO.sub.3. The increase in the rate of oxidation of the benign, gaseous dioxide form to the trioxide form results in the adsorption of greater amounts of acid sulfates and associated water onto the discharged particulates, increasing their mass.
The Snider, et al. report also discussed several other approaches, including the use of a fuel additive containing 80 ppm manganese and 20 ppm copper to reduce the regeneration temperature of the trap. While this was effective in reducing the particulate ignition temperature for a trap, this is not a consideration for flow-through oxidizers which do not trap particulates. No measurable reductions in carbon monoxide, unburned hydrocarbons or NO.sub.x were noted.
In a 1987 report, R. W. McCabe and R. M. Sinkevitch summarized their studies of diesel traps catalyzed with platinum and lithium, both individually and in combination. (Oxidation of Diesel Particulates by Catalyzed Wall-Flow Monolith Filters. 2. Regeneration Characteristics of Platinum, Lithium, and Platinum-Lithium Catalyzed Filters; SAE Technical Paper Series-872137) They noted that carbon monoxide conversion to the dioxide was negligible over the lithium filter, good for platinum, but good only initially for the combined catalyst. They further noted that platinum undergoes a reversible inhibition due to the presence of SO.sub.2, but in the presence of the lithium catalyst there is apparently a wetting of the platinum crystallites by Li.sub.2 O.sub.2.
The use of catalysts has taken many forms, but none have been found to be fully ,.satisfactory. While catalysts can be effective in reducing carbon monoxide and unburned hydrocarbons, they are either too easily fouled, have associated health risks, catalyze the oxidation of SO.sub.2 to SO.sub.3 (which then combines with water and adds to the mass of particulates), or have two more of these shortcomings.
The catalysts will typically be applied to a heat-resistant support and contain a combination of two or more catalyst metals--preferably selected from among platinum group metals such as platinum, palladium, and/or rhodium, transition metals such as cerium, copper, nickel and iron. Unfortunately, the general experience with combinations of catalyst metals, including palladium, platinum, and the even more-expensive rhodium, is that they lose their optimal activity after reasonably short periods of operation. Renewing the catalysts generally entails replacing the unit--an extremely costly undertaking. It would be desirable to simply and inexpensively enable the initial selective catalysis to provide oxidation of hydrocarbons without the formation of SO.sub.3 and continued operation for extended service.
While not related to flow-through oxidizers for diesel engines, Dalla Betta, Tsurumi, and Shoji disclose in U.S. Pat. No. 5,258,349, that graded palladium catalysts are sometimes necessary to provide a low light off temperature and avoid hot spots. In U.S. Pat. No. 5,216,875, Kennelly and Farrauto disclose that maintenance of combustion temperature is necessary to avoid inactivation of palladium oxide catalysts.
In addition, the art is replete with teachings that the production of pollutants such as unburned hydrocarbons can cause severe loss of catalytic activity. Also, sulfur and other compounds, such as chlorine, phosphorous, arsenic, lead, and the like, tend to poison or otherwise inactivate catalysts. The above patents and all of the those referred to or cited therein are specifically incorporated by reference in their entireties to show the structure, composition and operation of flow-through oxidation catalysts.
There is a present need for an improved means for rendering the exhaust from diesel engines more environmentally benign, and, especially to enable this without the rapid inactivation of expensive catalytic units or the production of harmful byproducts such as increased levels of sulfates. There is a need to enable the operation of a pass-through catalytic oxidizer over long time periods with continued catalytic activity and without excessive conversion of SO.sub.2 to SO.sub.3.